Ch. 9 Water Erosion and Deposition Earth Science

Section 1 – Surface Water Learning Objectives Identify the cause of runoff. Compare rill, gully, sheet, and stream erosion. Identify three different stages of stream development. Explain how alluvial fans and deltas form.

Section 1 – Surface Water A. Runoff – rainwater that flows across Earth’s surface; amount of runoff is affected by several factors: 1. Amount of rain 2. Length of time it falls 3. Steepness, or slope, of the land 4. Amount of vegetation

B. Erosion – the wearing away of soil and rock by water

1. Rill erosion – during heavy rain, a small stream forms and carries away soil, creating a groove called a channel. 2. Gully erosion – Heavy rains remove large amounts of soil and sediment from a rill channel, forming a gully.

3. Sheet erosion – Sheets of water flowing across the land pick up and carry away sediments. 4. Stream erosion – Water in a stream picks up sediments from the bottom and sides of its channel, making the channel deeper and wider.

C. River system – the network of groundwater and streams that come together to form a river 1. Drainage basin – the area of land from which a stream or river collects runoff

2. Mississippi River drainage basin – largest in the United States.

D. Stages of stream development: 1. Young stream – flows swiftly through a steep valley a. May have whitewater rapids and waterfalls b. Erodes the stream bottom faster than the sides

2. Mature stream – flows more smoothly through its valley a. Erodes more on its sides b. Curves form called meanders c. Carves a broad, flat valley floor called a floodplain.

3. Old stream – flows slowly through a floodplain it has carved

E. Flooding – When too much water enters a river system, it overflows its banks.

Catastrophic Floods (Add to notes) Channeled Scablands – Washington State

1. Dams – built to control the flow of water downstream 2. Levees – mounds of earth built along the sides of a river to prevent flooding

F. Deposition – when water slows down, it drops, or deposits, the sediments it is carrying. 1. Delta – formed by sediments that are deposited as water empties into an ocean or lake.

2. Alluvial fan – fan-shaped area formed by sediments deposited as water empties form a mountain valley onto a flat open plain.

Discussion Question Why do curves (meanders) form in mature streams?

Discussion Question Why do curves (meanders) form in mature streams? The speed of the water changes with the width of the stream channel  Shallow areas – water moves slower  Deep areas – water moves faster  If a deep area is near the side of the river, water will erode the side faster and form a curve

Section 2 – Groundwater Learning Objectives Recognize the importance of groundwater. Describe the effect that soil and rock permeability have on groundwater movement. Explain how groundwater dissolves and deposits minerals.

Section 2 – Groundwater A. Groundwater – Water that soaks into the ground and collects in the pores of the rock underlying the soil 1. Only 30% of all freshwater exists as groundwater.

2. Soil and rock are permeable if water can pass through the pore spaces. Example: sandstone 3. Soil and rock are impermeable if water cannot pass through them. Example: granite

4. Aquifer – a layer of permeable rock that lets water move freely a. Zone of saturation: the area where all pores in the rock are filled with water b. Water table – upper surface of zone of saturation Unsaturated zone Water table Saturated zone Ground water

B. Water table 1. Many people get their water from groundwater through wells. a. Groundwater flows into a well, and a pump brings it to the surface. b. Good wells extend deep into the zone of saturation, past the water table. c. Wells can dry up during dry seasons or if demand is too high.

2. Artesian wells – water rises to the surface under pressure

3. Springs – the water table is so close to the surface that water flows out.

4. Geyser – a hot spring that erupts periodically, shooting water and steam into the air.

C. The work of groundwater 1. Groundwater mixes with carbon dioxide to form carbonic acid. 2. Acidic groundwater moves through cracks in limestone, dissolving the rock 3. Gradually, the cracks in the rock enlarge until a cave is formed.

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a. Stalactites – calcium carbonate deposit that hangs from a cave’s ceiling b. Stalagmites – calcium carbonate deposit that forms on a cave’s floor

4. If underground rock is dissolved near the surface, a sinkhole forms.

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Discussion Question What happens when too much groundwater is removed from an area?

Discussion Question What happens when too much groundwater is removed from an area?  Wells will dry up…people will have to wait for the water table to rise by rainfall or from groundwater flowing from other areas of the aquifer.  The land level could sink from the weight of the ground above the now-empty pore spaces, possibly creating sinkholes.

Section 3 – Ocean Shoreline Learning Objectives Identify the different causes of shoreline erosion. Compare and contrast different types of shorelines Describe origins of sand.

Section 3 – Ocean Shoreline A. Shoreline forces – three major forces constantly change the shape of the shoreline. 1. Waves pound against shores, breaking rocks into smaller pieces. a. Move large amounts of sediment b. Collide with the shore at angles, creating longshore currents, which act like rivers of sand in the ocean.

2. Currents move sediment along the shoreline. 3. Tides carry sediment out to sea and bring in new sediment.

B. Rocky shorelines – steep shorelines 1. Rocks and cliffs are the most common features 2. Softer rocks erode away before harder rocks, leaving islands of harder rocks.

C. Sandy beaches – gently sloping shorelines 1. Beaches – deposits of sediment that are parallel to the shore. a. Made of rock and shell fragments b. Fragile because longshore currents constantly carry sand down the shore to form barrier islands, spits, sand-bars, and baymouth bars.

2. Barrier islands – fragile sand deposits that parallel the shore but are separated from mainland.

Discussion Question Why are beaches fragile?

Discussion Question Why are beaches fragile?  Beaches are made from deposits of sediment, which are constantly being carried away by waves, tides, and currents.